The interaction of macrophages with T Lymphocytes is a prerequisite for antigen and mitogen activation of lymphocyte proliferation and mediator production. Macrophages facilitate lymphocyte activation in part by producing a lymphocyte activation factor "LAF". We have investigated the effects of LAF containing supernatants of human mononuclear cells on cultures of mouse thymocytes. LAF not only enhanced the proliferation of thymocytes, but under some circumstances had inhibitory effects. In addition to this regulatory role, LAF also induced thymocytes to differentiate and become better stimulator cells in mixed leucocyte reactions. LAF induced immediate increases in cAMP content and adenyl cyclase activity of thymocyte membranes. Pharmacological agents that influence cyclic nucleotide levels also have modulating effects on thymocyte proliferation. Factors influencing macrophages to produce LAF are also being studied. In general, stimulants with adjuvant effects which activate macrophages and increase their tumoricidal capacity and reactivity to chemotactic stimulants, also induce them to produce more LAF. The mechanism of action, characterization and production of LAF and its role in the amplification of immunological responses are being investigated further.